The present invention relates to an internal combustion engine purge flow rate controlling apparatus.
Fuel is supplied to an internal combustion engine of an automotive vehicle from a fuel feed pipe and a fuel tank via injectors. In addition, evaporative fuel produced within the fuel tank is absorbed and stored in a canister, and part of the evaporative fuel so absorbed and stored in the canister is purged to an induction system of the internal combustion engine via a purge passage. A purge control valve is provided along the purge passage, and the opening of the purge control valve is regulated according to running conditions of the internal combustion engine so as to control the flow rate of evaporative fuel purged to the induction system.
When such purging is carried out, since the total volume of fuel supplied to the internal combustion engine is a sum of the volume of fuel that is supplied via the injectors (hereinafter, referred to as a “injected fuel volume”) and the volume of evaporative fuel that is purged to the induction system, in order to implement an air-fuel ratio control with good accuracy, the purge flow rate needs to determined based on the running conditions of the internal combustion engine so as to optimally control the volume of evaporative fuel so purged.
To this end, in the related art, a deviation between a target duty ratio set based on the running conditions (engine rotational speed and engine load) of the internal combustion engine (namely, the target purge flow rate) and a set duty ratio (a control duty ratio fed to an electromagnetic valve. Namely, the control purge flow rate) is calculated, and when the set duty ratio is smaller, then, a positive constant is added to the set duty ratio, whereas, when the set duty ratio is larger, then, the positive constant is subtracted from the set duty ratio, whereby the fluctuation in air-fuel ratio can be suppressed by gradually increasing or decreasing the purge flow rte (refer to, for example, Patent Literature No. 1).
[Patent Literature No. 1]
JP-B-3-21744 (page 3, FIG. 2)
In the related art, however, since the value added to or subtracted from the set duty ratio is the constant that is set in advance, in the event that a drastic change in running condition of the internal combustion engine varies the target purge flow rate largely, there is caused a problem that the control purge flow rate may delay in following the target purge flow rate and the control purge flow rate may vary largely due to a slight change in running condition of the internal combustion engine, whereby the air-fuel ratio becomes unstable and a desired output cannot be obtained, this leading to a deterioration in running properties of the internal combustion engine.